In nature, lignocellulosic biomass, such as wood and grass, is roughly consisted of 50% cellulose, 25% hemicellulose, and 20% lignin. Cellulose can be hydrolyzed into saccharides which are further used for producing cellulosic ethanol through fermentation. Because lignocellulosic biomass is water-insoluble, the current industrial methods for hydrolyzing biomass proceed mainly in a semi-continuous percolating reactor containing 0.4˜0.8% dilute aqueous sulphuric acid, at temperature of 180˜190 and under pressure of 12˜14 atmospheres [see, Nanjing Forestry Institute. Plant Hydrolysis Technology (Ed. in Chinese). Beijing: Agricultural Press, 1961.]. Reports about continuous production of sugars in a high-pressure continuous flow reactor were not found.
It is shown in most recent studies that high-pressure hot-water is a weak-polar solvent, presenting both acidic and basic. As a result, it can be used to dissolve biomass, and allow hydrolysis reactions to be performed in a homogeneous phase. It was found by Sasaki et al. that cellulose can be completely dissolved in water at 320 and a water density of more than 1000 kg/m3 [see: Sasaki M, Fang Z, Fukushima Y, Adschiri T, Arai K. Dissolution and hydrolysis of cellulose in subcritical and supercritical water. Ind. & Eng. Chem. Res. 2000; 39(8):2883-2890.]. Afterwards, it was further found by Ogihara et al. that the minimum temperature for completely dissolving cellulose at a water density ranging from 550 to 1000 kg/m3 is 320 at a water density of 850 kg/m3. [see: Ogihara Y, Smith Jr. RL, Inomata H, Arai K. Direct observation of cellulose dissolution in subcritical and supercritical water over a wide range of water densities (550˜1000 kg/m3). Cellulose. 2005;12:595˜606.]. The above findings all relate to pure water, and cellulose has a higher dissolving temperature (or pressure) and a lower hydrolysis rate and yield in pure water. On the basis of the above works, the inventor, after studies, found that addition of a minor amount of alkaline salts in pure water allows lignocellulosic biomass such as wood powders to be dissolved completely and to be undergone a homogeneous hydrolysis at 329˜367 . A Chinese patent was then filed accordingly (application number: 200710141265.3; CN101235095-A).
Because of the restriction of the condition back then, the above patent application does not involve the influence of acidic and extremely weak basic solutions on dissolution of cellulose. Also, how to completely dissolve and rapidly hydrolyze cellulose at a lower reaction temperature using a high-pressure continuous flow reactor is still a technical problem to be further solved.